Silver crystals in the form of granules, gauze, wire turnings, crystals, and the like have been used for many years to produce formaldehyde by the oxidative dehydrogenation of methanol. Silver crystals are especially suitable for this purpose since they are very selective and have little tendency to promote side reactions and the formation of by-products under reaction conditions which permit high overall yields. However, when using such crystals, the silver surface available for catalysis is relatively small compared to the remaining silver of the catalyst in the interior of the crystals which is not so available. Thus there has been a continuous effort to find a satisfactory catalyst wherein silver is deposited on an inert support for its more effective use in the production of formaldehyde from methanol.
Numerous prior art patents and publications describe silver on inert supports used as catalysts used in the production of formaldehyde from methanol. For example, U.S. Pat. No. 3,956,184, issued to Kruglikov et al on May 11, 1976, describes catalysts containing silver on supports such as silica gel, carborundum, corundum, pumice, diatomaceous earth, perlite and the like used for this purpose. Silver on an alumina support is shown in U.S. Pat. No. 3,959,383 issued to Northeimer on May 25, 1976. Catalysts comprising silver-gold alloys on various supports are described in U.S. Pat. No. 4,167,527 issued to Nielsen on Sept. 11, 1979.
Silver containing up to 10% of an oxide of barium, strontium and/or calcium and up to 8% of an oxide of indium on any of various supports such as silicon nitride, boron nitride, silicon carbide, silica and alumina are described as methanol oxidative dehydrogenation cataylsts in U.S. Pat. No. 4,045,369 issued to Cantiluppi on Aug. 30, 1977. Silver catalysts on support materials are described in U.S. Pat. No. 2,424,085 issued to Bergsteinsson et al on July 15, 1947. The support materials disclosed include elemental silicon, silicon carbide, or silicon-silicon carbide aggregates. These catalysts can be used in various processes such as oxidation, dehydrogenation, and the like. U.S. Pat. No. 3,948,997, issued Apr. 6, 1976 to Howe et al. describes a process for the vapor phase oxidation of .alpha., .beta.-diols such as ethylene glycol to .alpha., .beta.-diones such as glyoxal at elevated temperatures in the presence of a catalyst containing as essential constituents, one or more metals of Group Ib of the Periodic Table comprising copper, silver and gold, and one or more elements from Group IV a, comprising germanium, tin and lead, and Group V a, comprising nitrogen, phosphorus, arsenic, antimony and bismuth. The catalyst may be supported, if desired on an inorganic support material, for example, pumice and alumina. However, there is no specific disclosure in this patent of a lead-silver catalyst per se nor is there any suggestion that any of the catalysts disclosed generically or specifically, could be used for the oxidative dehydrogenation of methanol to formaldehyde.
Silver catalysts are also known for their use in the production of ethylene oxide from ethylene. U.S. Pat. No. 4,255,341 issued to Solomon on Mar. 10, 1981 describes ethylene oxidation catalysts including silver on supports such as .alpha.-alumina and silicon carbide among other supports. U.S. Pat. No. 4,242,235 issued to Cogmon et al on Dec. 30, 1980 describes ethylene oxide catalysts of silver on refractory supports such as .alpha.-alumina, silica-aluminas, silicon carbide, zirconia and graphite. However, none of these patents disclose or suggest supported lead-silver catalysts.